Polymer Molecular Weight Exercise 1

Avarege molecular weights Equations for number average and weight average molecular weights as well as polydispersity index are defined as follows: where Mi molecular weight of molecules i ni number of molecules with molecular weight i wi mass of the molecules with molecular weight i

2 A sample of polystyrene is composed of a series of fractions of different sized molecules. Calculate the number average and weight average molecular weights of this sample as well as the PDI. How would adding styrene oligomer change the average molecular weights? Added amount is 5wt.% of polymer mass and M=1000g/mol. Table 1. PS fractions. Fraction   weight fraction Molecular weight [g/mol] A 0.130 11000 B 0.300 14000 C 0.400 17000 D 0.170 21000

2a) Determine the number of moles in each fraction. Assume that the sample is 10 g in the beginning. Number of moles of the fraction is ni = wi / Mi. fraction wi [g] Mi [g/mol] ni [mmol] A 1.30 11000 0.118 B 3.00 14000 0.214 C 4.00 17000 0.235 D 1.70 21000 0.0810   wi = 10.0 g ni = 0.648 mmol

2a) Number average molecular weight: Weight average molecular weight: Polydispersity index:

2b) When 5.0 wt-% of styrene oligomer (fraction E) is added, the total mass and number of moles increase as follow: fraction wi [g] Mi [g/mol] ni [mmol] A 1.30 11000 0.118 B 3.00 14000 0.214 C 4.00 17000 0.235 D 1.70 21000 0.0810 E 0.50 1000   wi = 10.5 g ni = 1.10 mmol

2b) number average molecular weight weight average molecular weight   Polydispersity index